Anaemia means that the haemoglobin concentration and the so called haematocrit in blood are lower than normal. In most, but not in all, of the many types of anaemia, there is also a decrease in the number of red blood cells.

According to the European Commission a disease is rare when its population frequency is less than 5 cases in 10,000 individuals. The rare causes of anaemia (rare anaemias) are an example of rare diseases (RD). Up to date, about 90 rare anaemias have been described.